Transcript Wireless Communications and Networks

Cordless Systems
and Wireless Local Loop
Chapter 11
Cordless System
Operating Environments


Residential – a single base station can
provide in-house voice and data support
Office



A single base station can support a small office
Multiple base stations in a cellular
configuration can support a larger office
Telepoint – a base station set up in a public
place, such as an airport
11-2
Design Considerations
for Cordless Standards



Modest range of handset from base station,
so low-power designs are used
Inexpensive handset and base station,
dictating simple technical approaches
Frequency flexibility is limited, so the
system needs to be able to seek a lowinterference channel whenever used
11-3
Time Division Duplex (TDD)


TDD also known as time-compression
multiplexing (TCM)
Data transmitted in one direction at a time,
with transmission between the two
directions


Simple TDD
TDMA TDD
11-4
Simple TDD







Bit stream is divided into equal segments,
compressed in time to a higher transmission
rate, and transmitted in bursts
Effective bits transmitted per second:
R = B/[2(Tp+Tb+Tg)]
R = effective data rate
B = size of block in bits
Tp = propagation delay
Tb = burst transmission time
Tg = guard time
11-5
Simple TDD

Actual data rate, A:
A = B /Tb

Combined with previous equation:
 T p  Tg
A  2 R1 
Tb





The actual data rate is more than double the
effective data rate seen by the two sides
11-6
TDMA TDD

Wireless TDD typically used with TDMA


A number of users receive forward channel
signals in turn and then transmit reverse
channel signals in turn, all on same carrier
frequency
Advantages of TDMA/TDD:


Improved ability to cope with fast fading
Improved capacity allocation
11-7
DECT Frame Format






Preamble (16 bits) – alert receiver
Sync (16 bits) – enable receiver to
synchronize on beginning of time slot
A field (64 bits) – used for network control
B field (320 bits) – contains user data
X field (4 bits) – parity check bits
Guard (60 bits) – guard time, Tg
Preamble
Sync
A Field
B Field
X Field
Guard
11-8
DECT Phone
11-9
A Field Logical Control Channels





Q channel – used to broadcast general system
information from base station to all terminals
P channel – provides paging from the base
station to terminals
M channel – used by terminal to exchange
medium access control messages with base
station
N channel – provides handshaking protocol
C channel – provides call management for
active connections
11-10
B Field

B field transmits data in two modes


Unprotected mode - used to transmit digitized
voice
Protected mode - transmits nonvoice data traffic
11-11
DECT Protocol Architecture
DECT Protocol Architecture


Physical layer – data transmitted in TDMA-TDD
frames over one of 10 RF carriers
Medium access control (MAC) layer – selects/
establishes/releases connections on physical
channels; supports three services:




Broadcast
Connection oriented
Connectionless
Data link control layer – provides for the reliable
transmission of messages using traditional data
link control procedures
11-13
Differential Quantization

Speech signals tend not to change much between
two samples



Transmitted PCM values contain considerable
redundancy
Transmit difference value between adjacent
samples rather than actual value
If difference value between two samples exceeds
transmitted bits, receiver output will drift from the
true value

Encoder could replicate receiver output and additionally
transmit that difference
11-14
Differential PCM (DPCM)

Since voice signals change relatively
slowly, value of kth sample can be
estimated by preceding samples

Transmit difference between sample and
estimated sample


Difference value should be less than difference
between successive samples
At the receiver, incoming difference value is
added to the estimate of the current sample

Same estimation function is used
11-15
Adaptive Differential PCM (ADPCM)

Improve DPCM performance using
adaptive prediction and quantization


Predictor and difference quantizer adapt to the
changing characteristics of the speech
Modules



Adaptive quantizer
Inverse adaptive quantizer
Adaptive predictor
11-16
ADPCM Encoder
11-17
ADPCM Decoder
11-18
Subject Measurement
of Coder Performance



Subjective measurements of quality are
more relevant than objective measures
Mean opinion score (MOS) – group of
subjects listen to a sample of coded speech;
classify output on a 5-point scale
MOS scale is used in a number of
specifications as a standard for quality
11-19
Wireless Local Loop

Wired technologies responding to need for
reliable, high-speed access by residential,
business, and government subscribers



ISDN, xDSL, cable modems
Increasing interest shown in competing
wireless technologies for subscriber access
Wireless local loop (WLL)


Narrowband – offers a replacement for existing
telephony services
Broadband – provides high-speed two-way voice and
data service
11-20
WLL Configuration
Advantages of WLL
over Wired Approach



Cost – wireless systems are less expensive
due to cost of cable installation that’s avoided
Installation time – WLL systems can be
installed in a small fraction of the time
required for a new wired system
Selective installation – radio units installed
for subscribers who want service at a given
time

With a wired system, cable is laid out in
anticipation of serving every subscriber in a given
area
11-22
Propagation Considerations for WLL

Most high-speed WLL schemes use
millimeter wave frequencies (10 GHz
to about 300 GHz)



There are wide unused frequency bands
available above 25 GHz
At these high frequencies, wide channel
bandwidths can be used, providing high data
rates
Small size transceivers and adaptive antenna
arrays can be used
11-23
Propagation Considerations for WLL

Millimeter wave systems have some
undesirable propagation characteristics



Free space loss increases with the square of the
frequency; losses are much higher in millimeter
wave range
Above 10 GHz, attenuation effects due to
rainfall and atmospheric or gaseous absorption
are large
Multipath losses can be quite high
11-24
Fresnel Zone

How much space around direct path between
transmitter and receiver should be clear of obstacles?


Objects within a series of concentric circles around the
line of sight between transceivers have
constructive/destructive effects on communication
For point along the direct path, radius of first Fresnel
zone:
R
SD
SD
S
D
R


S = distance from transmitter
D = distance from receiver
11-25
Atmospheric Absorption

Radio waves at frequencies above 10 GHz
are subject to molecular absorption



Peak of water vapor absorption at 22 GHz
Peak of oxygen absorption near 60 GHz
Favorable windows for communication:


From 28 GHz to 42 GHz
From 75 GHz to 95 GHz
11-26
Effect of Rain

Attenuation due to rain



Presence of raindrops can severely degrade the
reliability and performance of communication links
The effect of rain depends on drop shape, drop size,
rain rate, and frequency
Estimated attenuation due to rain:
A  aR
b



A = attenuation (dB/km)
R = rain rate (mm/hr)
a and b depend on drop sizes and frequency
11-27
Effects of Vegetation




Trees near subscriber sites can lead to
multipath fading
Multipath effects from the tree canopy are
diffraction and scattering
Measurements in orchards found
considerable attenuation values when the
foliage is within 60% of the first Fresnel zone
Multipath effects highly variable due to wind
11-28
OFDM (Orthogonal FDM)




Split a data source of R bps into N
substreams of R/N bps.
Expand bit duration from 1/R to N/R, to
overcome intersymbol interference (ISI).
All substreams are transmitted over multiple
orthogonal subcarriers.
Orthogonality: the peaks of the power
spectral density of each subcarrier occurs at a
point at which the power of other subcarriers
is zero.
11-29
OFDM
11-30
OFDM
11-31
Multipoint Distribution Service (MDS)

Multichannel Multipoint Distribution
Service (MMDS)



Also referred to as wireless cable
Used mainly by residential subscribers and
small businesses
Local Multipoint Distribution Service
(LMDS)

Appeals to larger companies with greater
bandwidth demands
11-32
Advantages of MMDS



MMDS signals have larger wavelengths and
can travel farther without losing significant
power
Equipment at lower frequencies is less
expensive
MMDS signals don't get blocked as easily
by objects and are less susceptible to rain
absorption
11-33
Advantages of LMDS



Relatively high data rates
Capable of providing video, telephony, and
data
Relatively low cost in comparison with
cable alternatives
11-34
Communications Bands Allocation
11-35
WiMAX
802.16 Standards Development






WiMAX = Word Interoperability for
Microwave Access (AX)
Use wireless links with microwave or
millimeter wave radios
Use licensed spectrum
Are metropolitan in scale
Provide public network service to fee-paying
customers
Use point-to-multipoint architecture with
stationary rooftop or tower-mounted antennas
11-36
Coverage Comparison
11-37
802.16 Standards Development



Provide efficient transport of heterogeneous
traffic supporting quality of service (QoS)
Use wireless links with microwave or
millimeter wave radios
Are capable of broadband transmissions (>2
Mbps)
(802.16.1)
(802.16.3)
11-38
802.16 Standards Development
802.16(.1)
802.16a(.3)
802.16e
Completion
Dec. 2001
Jan. 2003
Q3. 2004
Spectrum
Channel
condition
10~66GHz
<11GHz
<6GHz
LOS
NLOS
NLOS
Modulation
QPSK,16QAM,
64QAM
QPSK,16QAM,64
QAM, OFDM 256
subcarrier
Same as 802.16a
20/25/28 MHz
1.25~20 MHz
Same with 802.16a
Bit rate
32~134Mbps
(in 28MHz channel)
75 Mbps (in
20MHz channel)
30Mbps (in 10Mhz
channel)
Cell range
1~3 miles
3~5 miles
1~3miles
Channel
length
11-39
Integrate 802.16 and 802.11
11-40
IEEE 802.16 Protocol Architecture
11-41
Protocol Architecture

Physical and transmission layer functions:




Encoding/decoding of signals
Preamble generation/removal
Bit transmission/reception
Medium access control layer functions:



On transmission, assemble data into a frame with
address and error detection fields
On reception, disassemble frame, and perform
address recognition and error detection
Govern access to the wireless transmission
medium
11-42
Protocol Architecture

Convergence layer functions:




Encapsulate PDU framing of upper layers into
native 802.16 MAC/PHY frames
Map upper layer’s addresses into 802.16
addresses
Translate upper layer QoS parameters into
native 802.16 MAC format
Adapt time dependencies of upper layer traffic
into equivalent MAC service
11-43
IEEE 802.16.1 Services







Digital audio/video multicast
Digital telephony
ATM
Internet protocol
Bridged LAN
Back-haul
Frame relay
11-44
IEEE 802.16.3 Services



Voice transport
Data transport
Bridged LAN
11-45
IEEE 802.16.1 Frame Format

11-46
IEEE 802.16.1 Frame Format
11-47
IEEE 802.16.1 Frame Format
11-48
IEEE 802.16.1 Frame Format

Header - protocol control information





Downlink header – used by the base station
Uplink header – used by the subscriber to
convey bandwidth management needs to base
station
Bandwidth request header – used by subscriber
to request additional bandwidth
Payload – either higher-level data or a MAC
control message
CRC – error-detecting code
11-49
MAC Management Messages






Uplink and downlink channel descriptor
Uplink and downlink access definition
Ranging request and response
Registration request, response and
acknowledge
Privacy key management request and
response
Dynamic service addition request, response
and acknowledge
11-50
MAC Management Messages





Dynamic service change request, response,
and acknowledge
Dynamic service deletion request and
response
Multicast polling assignment request and
response
Downlink data grant type request
ARQ acknowledgment
11-51
Physical Layer–Upstream Transmission



Uses a DAMA-TDMA technique
Error correction uses Reed-Solomon code
Modulation scheme based on QPSK
11-52
Physical Layer–Downstream Transmission

Continuous downstream mode




For continuous transmission stream (audio, video)
Simple TDM scheme is used for channel access
Duplexing technique is frequency division duplex
(FDD)
Burst downstream mode



Targets burst transmission stream (IP-based traffic)
DAMA-TDMA scheme is used for channel access
Duplexing techniques are FDD with adaptive
modulation, frequency shift division duplexing (FSDD),
time division duplexing (TDD)
11-53